A fuel cell is an electrochemical cell that converts chemical energy directly into electrical energy through a chemical reaction between an electrolyte, a fuel, and an oxidizer. The use of fuel cells is becoming of ever-greater interest as a potentially cleaner and more efficient manner of producing electrical energy, in comparison to combustion-based power generation processes and systems. For operating a fuel cell, generally two gaseous raw materials or input materials are needed: namely hydrogen (H2) supplied to the anode side of the fuel cell, and oxygen (O2) supplied to the cathode side of the fuel cell. Depending on the fuel cell type, these gases can be supplied as pure gases in molecular form, or components of gas mixtures, or recovered from other chemical compounds through so-called reforming processes (for example hydrogen being obtained by reforming hydrocarbons provided in a fuel).
In various applications, for example in mobile applications such as in vehicles, the oxygen input proportion required by the fuel cell is recovered from the surrounding ambient environmental air, which is a mixed gas comprising approximately 18% oxygen (O2), 78% nitrogen (N2), and 4% other gases (CO2 and trace gases). This means that only 18% of the total input gas is actually available for use in the fuel cell, while approximately 82% of the input gas supplied to the oxygen side of the fuel cell is not useable in the fuel cell reaction process. That necessarily leads to inefficiencies and undesired side effects in the process. In the case of high temperature fuel cells, which operate at temperatures in the range of about 600 to 1000° C., this further leads to undesired thermochemical reactions, such as the formation of nitrogen oxides (NOX). Furthermore, when the hydrogen (H2) needed for the fuel cell is to be recovered from a hydrocarbon fuel, such as a mineral oil, the pertinent reforming process will simultaneously generate gases that are not useable by the fuel cell and thus are emitted as exhaust gases (CO, CO2, CXHY). These undesired byproduct gases become pollutant components of the output exhaust gas.
In view of the above considerations, it is desirable and needed in the field of fuel cells, to increase the efficiency and reduce the exhaust emissions in the operation of the fuel cell.
German Patent Publication DE 198 21 952 C2 discloses an energy generating or supply unit onboard an aircraft, as a substitute for a main engine generator, a so-called auxiliary power unit (APU), a RAM air turbine (RAT), or a NiCd battery pack. The disclosed energy supply unit includes a fuel cell for generating an electrical direct current. The air needed by the fuel cell is provided by the exhaust air of the aircraft air conditioning plant or by external ambient air from outside the aircraft. Furthermore, water for the water supply of the aircraft is recovered from the fuel cell exhaust gases, and the remaining fuel cell exhaust gases are then emitted to the outside environment surrounding the aircraft. The hydrogen emitted by the fuel cell is also emitted to the ambient environment outside of the aircraft.
German Patent Publication DE 199 11 018 C1 discloses an auxiliary engine for an aircraft, and particularly a so-called auxiliary power unit (APU), which includes a gas turbine engine comprising a combustion chamber, a compressor, and a turbine. A work-output compressor coupled to the engine serves to generate compressed air for use onboard the aircraft. Additionally, the APU package further includes a fuel cell for generating electrical energy for use onboard the aircraft.
British Patent Publication GB 2,338,750 A discloses a jet engine with an integrated fuel cell. The fuel cell in this reference is used in a conventionally typical manner as generally discussed above. This reference, as well as the above mentioned references, do not disclose or suggest any reduction process by which the content of CO and/or NOX in the exhaust gas of a high temperature fuel cell can be reduced, or in which the hydrogen contained in the exhaust gas is further provided as a fuel to be utilized in a low temperature fuel cell. Thus, the above references have not addressed or suggested solutions to the problems to be addressed by the present invention.
Separately from any considerations regarding the structure or operation of fuel cell systems, it is generally known to use oxygen enrichment devices or oxygen generators to selectively separate an input flow of air into an output flow having an enriched oxygen content and an exhaust or byproduct flow having a reduced oxygen content. For example, in the context of aircraft systems, it is known to provide an oxygen enrichment device comprising a container with two molecular sieves arranged opposite each other therein. This device is operated by alternating the throughflow direction in this container, for enriching the oxygen content of an output gas flow, to be provided as an oxygen supply for the passengers of the aircraft, in a so-called OnBoard Oxygen Generating System (OBOGS). It is also known to use systems based on ceramic materials, and systems using the influence of electrostatic charges, for enriching the oxygen content of an output gas flow.